Classically, the endocrine vitamin D system is essential for maintaining calcium and phosphate homeostasis. Sufficient vitamin D levels promote bone health and prevent osteopathies, such as rickets and osteomalacia. However, the discovery of an intracrine vitamin D pathway alluded to other roles for vitamin D in regulating the immune system. Specifically, research on the innate immune response to Mycobacterium tuberculosis infection demonstrated a vitamin D-mediated antimicrobial response. Here, we continued to investigate the role of vitamin D in the immunobiology of mycobacterial infection. In Chapter 1, we studied mycobacterial infection using leprosy as a disease model. We demonstrated that induction of the type I IFN gene program during Mycobacterium leprae infection is an active immune evasion mechanism used to subvert the vitamin D-dependent antimicrobial response through the suppression of the vitamin D activating enzyme, CYP27B1. Moreover, our studies comparing macrophages associated with each of the two poles of the leprosy disease spectrum—lepromatous leprosy (L-lep) and tuberculoid leprosy (T-lep)—provided additional insight into susceptibility versus resistance to infection. T-lep-associated macrophages with elevated CYP27B1 expression and activity at baseline were resistant to type I IFN-mediated suppression and capable of inducing an antimicrobial response, whereas L-lep-associated macrophages were not. Taken together, these results demonstrate the importance of control of the vitamin D-dependent antimicrobial response in the host-pathogen interaction. In Chapter 2, we focused on markers of host defense to identify factors which confer protection against mycobacterial infection. These studies revealed that IL-32 is associated with IL-15- and IFN-γ-induced host defense networks. IL-32 is also necessary and sufficient to induce CYP27B1 activity and the downstream antimicrobial response. These studies pinpoint IL-32 as an important regulator of vitamin D-dependent host defense. Finally, in Chapter 3, we studied the effects of 1,25D on the iron regulatory protein, hepcidin, which regulates iron homeostasis by binding ferroportin and preventing the export of iron. Our results showed that 1,25D suppresses hepcidin expression in monocytes. These findings are important in the context of mycobacteria which are intracellular pathogens that rely on scavenging iron from the host to survive. Therefore, regulation of hepcidin may be another key battleground in the host-pathogen interaction.
Human host defense against mycobacteria depends on a functioning immune system. While it is currently established that Th1 cells are major players in host defense against mycobacteria, other cell types such as Th17 cells also correlate strongly with the protective forms of disease. However, the role of Th17 cells in the context of intracellular infection are incompletely understood. Recent work has shown that Th17 cells can secrete an antimicrobial protein IL-26, which can directly lyse extracellular bacteria. Given the established role of antimicrobial peptides against mycobacterial infection, we decided to further investigate IL-26 and determine whether it is antimicrobial against mycobacteria. We find that IL-26 can be secreted by PBMCs and purified T cells in response to IL-1β in the absence of T cell receptor activation. This process is also more rapid than TCR stimulation. Among helper T cells, we show that IL-1RI expression was necessary for this response and identified IL-1RI+ Th17 cells as a cell type that can secrete IL-26 in response to IL-1β. Furthermore, we establish that IL-26 secreted in response to IL-1β is functionally antimicrobial.
We also examined whether IL-26 is antimicrobial against intracellular bacteria. We find that IL-26 is differentially expressed between clinical forms of leprosy, with higher expression in tuberculoid leprosy, the resistant form of the disease, as compared to lepromatous leprosy, the progressive form of the disease. Incubation of IL-26 with M. leprae and attenuated M. tuberculosis strain H37RA led to dose dependent killing, both with bacteria in culture and, importantly, while the bacteria resided within infected macrophages. Additionally, we find that IL-26 treatment of infected macrophages stimulates the autophagy response and enhances phagolysosome fusion in a STING dependent manner. Altogether, this work uncovers a novel mechanism by which Th17 cells contribute to defense against mycobacterial infection.
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